Ok so most have acknowledged the fact that victimless crimes don't actually exist. Good, that means I don't need to argue that point alone should the need arise.
No I wouldn't turn them in, except as a last resort. Drugs, they're bad, I'd talk to them about it, talk to people they know about it, try and convince them of their foolishness, etc.
And by that time if they still decide that ruining their mind, body and life in general is a grand idea then they wouldn't be my friend any more and I would happily report their behaviour to the authorities.
I don't take well to being ignored, and I don't make a habit of remaining friends with fools. Hence the reason I had a falling out with my friend who would cut himself, someone who thinks self harming is something clever and refuses to listen to arguments to the contrary is not someone I want to be around.
Note: he's better now, we aren't as close as before but we've talked since and are on friendly terms.
Note: I wouldn't report someone for prostitution I don't see the harm, if they were married/had a girlfriend then I would have to think about whether I tell her, that might have something to do with how well I know her.